Gas turbine engines typically include cooling systems for limiting the temperatures experienced by relatively hot rotor components such as, for example, turbine blades. The turbine blades extract work from hot combustion gases and, therefore, typically include internal cavities and cooling air apertures through the walls thereof through which relatively cool compressor discharge air is channeled for cooling the blades. The cooling air must be provided at suitable pressures and flow rates for adequately cooling the turbine blades and maintaining acceptable backflow margin. Backflow margin is defined as the difference between the cooling air pressure inside the blade and the pressure of the hot combustion gases which flow over the blades. It is preferable that a positive backflow margin be maintained so that combustion gases are not ingested through the apertures and into the blades, which might lead to a reduction in blade life.
The cooling air is typically provided by diverting a portion of the compressor discharge air into the cooling system. However, energy losses and a net reduction in engine thermal efficiency results when any compressor air is diverted from its intended purpose for combustion. The prior art includes many turbine cooling systems, including systems which modulate by throttling the amount of cooling air required in accordance with the various modes of operation of an engine. However, although prior art cooling systems provide for cooling of rotor components, they do so at different levels of efficiency.